High speed printers typically require positioning servo feedback control systems for controlling the position of the print head, a printing mechanism such as a daisy wheel, the ribbon, and the paper platen. Control systems which are used in high-speed, true-font, moving-head, stop-to-print printers must have the ability to move each of these elements at a high speed from one position to another precise position and critically damp the motion near its end of travel. Such positioning control systems are useful for moving the devices at a high rate of speed between two precisely defined points. Therefore, such control systems are distinguished from mere velocity control servo systems because velocity control servo systems, such as those used to control tape speed in a tape recorder, merely control the velocity of the tape to a desired constant value, and are not concerned with the position of the tape.
Positioning feedback control systems typically operate a servo mechanism by accepting an input defining a commanded position of the servo mechanism, computing a position error from the difference between the commanded position and the actual position of the servo, and subtracting from this computed position error the sensed velocity of the servo mechanism. These control systems are concerned primarily with reaching the commanded position in the shortest possible time without overshooting the commanded position at the end of travel. The time of travel may be optimally minimized by selecting a velocity profile of a particular shape. Specifically, it has been found that linear velocity profiles of the servo are preferable.
However, just prior to bringing the servo motion to a complete stop, an exponential or non-linear velocity profile is preferable as the velocity of the servo approaches zero at its end of travel. It is well known that a velocity feedback in the feedback control system which is proportional to the square of the velocity yields a linear velocity profile during deceleration. Depending upon the characteristics of the hardware and the servo mechanism, the most desirable exponent of the velocity might be a number different from two.
It is well known that in analog feedback control systems, it is very difficult to obtain velocity feedback which is proportional to the square of the velocity, and it is nearly impossible to obtain velocity feedback which is proportional to velocity raised to a power slightly different from two.
Despite these disadvantages of analog feedback control systems, the substitution of a digital feedback control system in place of an analog control system results in a loss in the advantages which are inherent in an analog feedback control system. Such advantages include the continuous nature of the control exercised by the analog system over the servo mechanism and also the inherent ability of the analog system to generate an exponential deceleration near the end of servo travel due to the naturally linear velocity feedback of the analog control system.
In the prior art, combination of both a digital control system and an analog control system in one position servo control system has necessitated the use of additional control devices to permit both the digital and the analog control systems to interface with the same servo system. For example, a switch which switches the servo between the analog system and the digital system has been thought to be necessary.